Apply Opaque blend mode to all object at once
I just learned how to do this when trying to figure out an answer for you. Paste and run the code in the text editor after selecting all the objects you want to change.

import bpy

context = bpy.context

for o in context.selected_objects:

    o.active_material.blend_method = 'OPAQUE'


I made a little add-on to make a helper panel to do this, not very fancy but if it is useful then that is great. https://github.com/artistCDMJ/opacity_helper/archive/master.zip